High current density brine electrolysis

ABSTRACT

Described is a method of electrolyzing brine in a mercury cell, at applied current densities of at least 6 amperes per square inch, employing as the anode a titanium substrate bearing on at least a portion of the surface thereof a mixed oxide coating of from 30 to 90 percent stannic oxide, from 1.0 to 10 percent antimony oxide, from 1.0 to 50 percent of at least one platinum group metal oxide, and from 0.5 to 30 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides.

United States Patent [1 1 Kolb et al.

[451 Feb. 19, 1974 HIGH CURRENT DENSITY BRINE ELECTROLYSIS [21] Appl. No.: 352,418

[52] US. Cl. 204/99, 204/128, 204/290 F [51] Int. Cl. C01d 1/06, B23p 3/00, C01b 7/06 [58] Field of Search 204/99, 128, 290 F [56] References Cited UNlTED STATES PATENTS 10/1972 Entwisie et a1. 204/290 F 6/1972 King et al. 204/290 F Primary Examiner-G. L. Kaplan Assistant Examiner-R. L. Andrews Attorney, Agent, or Firm-Timothy E. Tinkler [57] ABSTRACT Described is a method of electrolyzing brine in a mercury cell, at applied current densities of at least 6 amperes per square inch, employing as the anode a titanium substrate bearing on at least a portion of the surface thereof a mixed oxide coating of from 30 to 90 percent stannic oxide, from 1.0 to 10 percent antimony oxide, from 1.0 to 50 percent of at least one platinum group metal oxide, and from 0.5 to 30 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides.

5 Claims, No Drawings 1 HIGH CURRENT DENSITY BRINE ELECTROLYSIS BACKGROUND OF THE INVENTION The electrolysis of aqueous sodium chloride solutions, i.e., brine, in electrolytic cells employing a flowing mercury cathode is widely practiced. Basically, the process comprises passing a direct current between essentially parallel anodes and cathodes through the brine, thereby liberating chlorine at the anode and depositing sodium on the mercury as an amalgam.

The apparatus employed for this purpose is quite expensive and a primary concern in commercial operation is to obtain the maximum amount of production per unit of floor space. One method of so doing is to increase the anode current density at which the cell operates. However, with the graphite anodes originally employed, the maximum current density that could be employed without drastically reducing the useful life of the anode was on the order of 5.0 asi. Even with the advent of dimensionally stable anodes, e.g., platinumcoated titanium, while operation at higher current densities was at least theoretically possible it was soon found in practice that these anodes rapidly passivated, that is, the operating voltage exceeded that at which practical operation was possible. The apparent cause of this passivation was brine depletion across the cell, oxygen evolution increasing with decreasing brine concentration.

STATEMENT OF THE INVENTION Therefore, it is an object of the present invention to provide a method for electrolyzing brine in a mercury cell at high current densities.

It is a further object of the present invention to provide a method for the high current density operation of mercury type cells for the electrolysis of brine wherein a low anode wear-rate and an extended anode life may be achieved.

These and other objects of the present invention will become apparent to those skilled in the art from the specification and claims that follow.

It has now been found that, in a process for the electrolysis of brine in a cell employing a flowing mercury cathode, an improvement consists essentially of employing as the anode a titanium substrate bearing on at least a portion of the surface thereof a mixed oxide coating of from 30 to 90 percent by weight stannic oxide, from 1.0 to percent antimony oxide, calculated as Sb O from 1.0 to 50 percent of at least one platinum group metal oxide, and from 0.5 to 30 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of tin to antimony oxides be between 85:15 and 95:5, and applying current to said anode at a rate of at least 6 amperes per square inch.

The foregoing improvement is particularly effective in the instance where the valve metal oxide is TiO, and the platinum group metal oxide is a combination of RuO,and IrO,.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The mercury cells to which the practice of the present invention may be applied may be of the horizontal, vertical, or inclined plane type, all of which are well known to those skilled in the art. Such cells and their operation are described, for example, at Kirk-Othmer,

Encyclopedia of Chemical Technology, Vol. 1, pages 688-695 (2nd Edition, 1963).

Typically such cells operate on aqueous sodium chloride solutions having concentrations at or approaching saturation (brine), e.g., 325 grams per liter, although sea water has been successfully electrolyzed in some instances. The distance between the parallel anodes and mercury cathode of the cell is kept quite low, often on the order of 0.1 to 0.3 inch. Such cells are operated at temperatures up to the boiling point of brine and at sodium-mercury amalgam concentrations on the order of from 0.1 to 0.15 percent, the latter factor being controllable to some extent by regulating the mercury flow rate. Conventional operation is at anode current densities on the order of 3 to 4 asi and up to 6 asi.

According to the present invention, the amount of current supplied to the cell is at least 6 amperes per square inch of anode surface, preferably 6 to 10 asi. At such increased current densities, the brine becomes severely depleted during its passage from inlet to outlet thereby increasing the amount of oxygen evolved at the anode surface and normally contributing to the rapid passivation of same.

However, this tendency toward passivation is greatly reduced when the anode comprises a titanium substrate bearing on at least a portion of the surface thereof a mixed coating of the oxides of tin, antimony, at least one platinum group metal, and a valve metal selected from the group titanium and tantalum. Such anodes also exhibit a long life, i.e., a low platinum group metal wear-rate per ton of chlorine.

The conductive substrate is generally titanium, although a more conductive material, such as copper or aluminum, bearing a surface of titanium may be employed. Additionally, layers on the substrate intermediate the titanium and the coating, such as those described in US Pat. No. 3,711,397, are contemplated. The configuration of the substrate may vary considerably but it is generally in the form of a sheet, particularly a foraminous sheet, such as expanded mesh.

What may be considered the first of the components in the coating composition is stannic oxide, preferably present in the form of crystalline SnO and employed within the range of from 30 to 90, especially 30 to 50, percent by weight of the total coating composition.

The antimony oxide component enters into the tin oxide crystal lattice, rendering same more electrically conductive. Although theantimony is present in an indeterminate oxide form owing to its entrance into the stannic oxide crystal lattice, it is expressed for convenience sake as Sb,O Thus, on this basis, the antimony oxide is present within the range from 1.0 to 10, preferably 4.0 to 8.0, percent by weight.

The foregoing ranges of tin and antimony oxides are further qualified by the proviso that they be present, respectively, in the range, on a mole ratio basis, of 95:5 to :15, especially :10. In this fashion, there is ob tained the desired doping effect of the antimony on the tin oxide without the presence of an excess separate phase of antimony oxide.

The third component of the mixed coating is at least one platinum group metal oxide, by which term it is intended to include the oxides of platinum, palladium, ruthenium, iridium, rhodium, and osmium, especially those of ruthenium and iridium. These platinum group metal oxides are present for the most part in their most acid. The concentration of the metals in thesolution 3 4 highly oxidized state and within the range of from 1.0 sition is incomplete, small amounts of salts may remain it 7 to 50, especially 20 to 40, percent by weight. An espewithout detrimental effect in the coating, for example, cially preferred anode is one the coating of which consmall amounts of chloride in the primarily oxide coattains a combination of RuO and IrO ing.

The final component is a valve metal oxide selected In order that those skilled in the art may more readily from the group consisting of titanium and tantalum oxunderstand the present invention and certain preferred ides. Whereas the titanium is present in the form of embodiments by which it maybe carried into effect, TiO and is essentially crystalline (rutile) in nature, the following specific example is afforded. when tantalum is em loyed, a generally amorphous tantalum oxide result: Therefore, although it is ex- 10 a a A e pressed as Ta 0 it is understood that mixtures of tan- Four anodes were prepared from the following Sohb talum oxides may in fact be present. In general, Ta O is tions. preferred. The amounts of a valve metal oxide em- Anodel 50m1n butano1 125 gSnC]4.5H2O, 0.91 ployed are generally within the range of from 0.5 to 30 g s c and 1 g c 14 0 33% Ru) Percent y weight, especially 15 to 25 percent 15 Anode 2 45 ml ethanol, 5.0 g orthobutyl titanate,

Thus, a preferred electrode comprises an expanded 1,1 g sbc1,, 15.1 g SnCl -5H O, and 7.6 g R1101, titanium metal substrate bearing a coating containing H 0 3 Ru). about 47 Percent t Percent z a p Anode 3 s0 ml n-butanol, 12.5 g SnCL'SH O, 0.91 cent Ru0 4.5 percent lrO and percent Ta Or- 20 g SbCI, 7.0 g orthobutyl titanate, and 1.1 g

While many of the variety of methods known for pro- R Ch- H Q (3 %R ducing mixed metal oxide coatings may be employed, Anode 4 .45 1 ethanol, 45 g c H g s a the preferred method of preparing the multicomponent 15 1 g S ch-511 0, and 7.6 g Rucl 'xll O coating composition on the titanium substrate is by de- (38%R position from a solution of the appropriate thermo- 25 Each anode i prepared b l i i coats f h chemically decomposable salts- For example, it is deslr' solution by brush to a clean titanium metal mesh with able to paint or brush an acidified alcoholic solution of h i i i between h t, fi t t 110 C for 3 said salts onto the substrate, followed by drying at minutes f ll ed b 7 minutes t 500C C for from 3 to especially 5, minutes and These are employed as anodes in a horizontal merfinally by aking in an xidiz g atmosphere, g-, cury cell spaced 0.14 inch above and parallel to a merat to especially for from 5 t0 cury cathode flowing at a rate of 450 ml/minute. The especially about 7, mlnhtes This Procedure y then electrolyte is a 310 g/l brine solution having a pH be repeated any number of times until the desired coatwithin the range of 33-6 and a. temperature of about 70 ing thickness is obtained, for example, 6 to 10 coats. C. To establish the wear-rate of the anodes, electrolysis The preferred solvents for the thermally decomposable 35 is continued. at 6 amperes per square inch for 500 salts arethe lower alkanols such as ethanol, propanol, hours, the loss being determined by weight differential.

amyl alcohol, and especially n-butyl alcohol, although Efficient low voltage operation is achieved throughout other solvents including water may be employed. To the test. Results, together with the composition of each these solvents there is generally added from 0 to 50 anode coating calculated on an oxide basis, appear in percent by volume of an acid, such as hydrochloric the following table.

TABLE Anode SnO,% Sb,0,% RuO,% TiO,% Ta,0,% Wear-Rate g/ton Cl from which the coating composition is to be derived the added valve metal oxide, exhibits the highest wearranges between about 50 to 200 grams per liter. The rate. When tantalum is empl y (Anode 0r Using salts employed are generally any thermally decomposrelatively Smal amou s o ti (A the able inorganic or organic salt or organic ester of the e esults are Obtained. metals in question such as the chlorides, nitrates, alkoxe C aim:

ides, alkoxy halides, resinates, amines, and the like. 1. In a P ocess for the electrolysis of brine in a cell Specific and illustrative examples include potassium with a flowll'lg 'y cathode, the improvement that hexachloroplatinate, hexachloroiridic acid, ruthenium Consists n ially o employing as the anode a titatrichloride or tribromide, orthobutyl titanate, antimony l substrate bea ing on at least a portion of the surtrichloride or pentachloride, and stannic chloride or e e eof a mixed Oxide coating of from 30 to 90 dibutyl tin dich]0ride percent by weight SnO from 1.0 to 10 percent anti- It will be understood by those skilled in the art that y oxide, Calculated as z a from L0 to 50 P it is possible to use a number of combinations of precent of at least one Platinum group metal oxlde, and

formed oxides of the various component metals and from to 30 Percent of a Valve metal OXlde elected salts of the remaining materials, although it is generally from a group consisting of tltahlum and tantalum believed that preformed valve'metal oxides should not ides, with the Proviso that the mole ratio of tin to antibe employed nor should separately preformed tin and molly oxides is between 85115 and 9515; na pp y hg antimony oxides be used. Further, if thermal decompoa ijlii ii jfisl5WPPYB From the table, it is evident that Anode 1, without square inch.

metal oxide is Ta O and is present within the range of 15 to 25 percent.

5. The improvement of claim 1 wherein the coating contains from 30 to 50 percent SnO 4 to 8 percent antimony oxide, 20 to 40 percent of at least one platinum metal oxide, and 15 to 25 percent Ta O 

2. An improvement as in claim 1 wherein the anode current density is between 6.0 and 10.0 amperes per square inch.
 3. The improvement of claim 1 wherein the platinum group metal oxide is a mixture of RuO2 and IrO2.
 4. The improvement of claim 1 wherein the valve metal oxide is Ta2O5 and is present within the range of 15 to 25 percent.
 5. The improvement of claim 1 wherein the coating contains from 30 to 50 percent SnO2, 4 to 8 percent antimony oxide, 20 to 40 percent of at least one platinum metal oxide, and 15 to 25 percent Ta2O5. 